Electric heaters for heating fluids have the advantages of being easy to install, because they require no exhaust for burned gases, clean, because there is no burned gas exhausted from a burner, and convenient, because electric current is readily available without the refilling required for propane and oil. Electric heaters do not require cleaning of burners and exhaust ducts and do not need the fairly expensive types of safety shut-offs of gas and oil fueled equipment.
Most electric heaters for liquids have electrical resistance heating elements that are suitably housed, usually in copper or aluminum tubes, and electrically insulated from and mechanically supported within the tubes by ceramic cores. This type of heating element is simply immersed in a vessel through which the liquid to be heated is passed. Some heaters for liquids use exposed electrodes that impress a voltage across the liquid, in which case the liquid itself is a resistance and is heated by the current passing through it.
Immersible electrical resistance heating elements are relatively costly to make and are subject to corrosion and to accummulation of deposits from the fluid due to direct contact with the fluid being heated. They also provide, individually, only a small surface area for heat transfer to the fluid. For rapid heating of a fluid a cluster of many elements is needed in order to provide a large heat transfer area. Baffles are often provided to direct the fluid flow back and forth across the cluster to enhance heat transfer. So-called "instantaneous" heaters, in which there is very little storage capacity, have very costly immersible heating units made up of many U-shaped elements or long coil elements and baffles.
There are various types of electric space heaters. For example, there are the radiant types in which the resistance elements are suitably mechanically supported in an open frame in front of a reflector and are physically protected, often inadequately, by a grid in front. A blower may be provided to induce heat transfer to the space by convection. Recently, heaters having resistance elements carried in transparent tubes have become popular. Many of the electric space heaters currently on the market are hazardous, because foreign objects can get close enough to a hot surface to ignite the object. Also, someone who accidentally touches certain parts of the housing can receive a painful burn.